Brother and sister Marty and Mitzie are two of a kind. Already victims of Vicky’s ‘babysitting’, getting caught up in Timmy’s Fairly Odd Summer can only be an improvement for these two. They’re good kids that are no strangers to getting in trouble, but this time they’ll need to help out Timmy and his fairies on this summer adventure!
Marty and Mitzie Mulligan are siblings with rich parents that never seem to have time for them. They often end up being babysat by Vicky while their parents are away because of work.
Being siblings, they both have similar features, like both being pale skinned red-heads. Marty has short hair, and his face is heavily covered in freckles. Mitzie has longer hair, done up in pigtails, with a pink floral print dress.
Marty and Mitzie, two rich kids, are in a limo with their parents, who are planning a family trip to Hawaii. As always happens for them, "a thing" makes it unable for Mr. and Mrs. Mulligan to go, but they decide to let their kids go anyway. Due to being busy, the parents hire Vicky to babysit them instead, much to their disappointment, as Vicky is still as mean as she always was.
Later in the movie, Tootie, alongside Marty and Mitzie, follow Foop and Denzel Crocker to a volcano where they plan to destroy the Abracadanium Stone affecting all the Fairy Magic by throwing it into the lava. When Crocker gives up doing it, due to the element's kindness affecting him, Foop traps him - with the Abracadabrium in hands. He manages to trap Tootie, Marty, Mitzie and all the fairies on a rock in the middle of the lava, stating that the rock would melt, destroying them, and the magical stone.
Timmy manages to save the day, but due to what happens in the climax, becomes a fairy himself, no longer needing Cosmo and Wanda. By the end of the film, Cosmo and Wanda are assigned to the siblings, as Timmy no longer needs the fairies himself.
Spoilers end here.
Marty and Mitzie are similar to Tammy and Tommy Turner, being a pair of siblings who get Cosmo and Wanda in the future, as a proposed end of the series.